New DNA tests suggest the owner of a British fertility clinic may have fathered as many as 600 children, while keeping his donations a secret. And in an even stranger twist, one of his newly discovered offspring says the man’s belief in eugenics may have been behind the decision.

The Telegraph reports that Bertold Wiesner and his wife operated a fertility clinic in the 1940s and were responsible for helping more than 1,500 families conceive. Two of the children conceived through clinic donations, Barry Stevens and David Gollancz, took part in DNA testing that revealed both were conceived using Wiesner’s sperm.

“Using standard figures for the number of live births which result, including allowances for twins and miscarriages, I estimate that he is responsible for between 300 and 600 children,” Gollancz told the paper.

Eighteen people conceived through the clinic between 1943 and 1962 were tested, with about two-thirds of them matching Wiesner’s DNA. However, it’s nearly impossible to determine exactly how many of the 1,500 children came directly from Wiesner, since his wife Mary Barton reportedly destroyed the records from the clinic. Wiesner himself died in the early 1970s.

Stevens told the Toronto Star that because of the donations he might have as many as 1,000 relatives he has never met. In addition, the Star reports that Austrian-born Wiesner was a “Communist sympathizer,” who belonged to a club of “left-wing biologists” who supported experimental medical techniques. The Daily Mail reports that the British House of Lords once described Wiesner’s work as “the work of Beelzebub.”

U.K. law prohibits “bulk donations” in order to reduce the possibility of unintended incest if couples from the same donor met and produced children. British donors can now make up to 10 such donations. There are no similar federal laws in the U.S.